It has heretofore been known that an effective aberration correction is attained by providing a lens with a desired refractive index distribution (gradient refractive index). Particularly, a radial-type refractive index distribution in which a refractive index is changed depending on a distance from an optical axis of the lens achieves a light collecting action by the refractive index distribution and exhibits a great effect on the aberration correction.
U.S. Pat. No. 4,457,590 discusses a single lens having the radial-type refractive index distribution. The refractive index of the single lens changes according to a fourth-order power series expansion formula depending on the distance in a radial direction from the optical axis, thus correcting spherical aberration and coma aberration well by maintaining coefficients of the power series expansion formula to desired values.
U.S. Pat. No. 5,359,456 discusses an example of using a lens having the radial-type refractive index distribution as a part of an endoscope objective lens. The lens includes, in order from the object side, a front group divergent system having a negative power and a rear group convergent system having a positive power, and at least one of the divergent system is a gradient index lens.
U.S. Patent Application Publication No. 2008/0239510 discusses an optical element using a nanocomposite material obtainable by mixing an optical resin (base material) with minute particles (nanoparticles). It is discussed in U.S. Patent Application Publication No. 2008/0239510 that a nanocomposite resin material of which a refractive index is improved by the nanoparticles is suitable as a high refractive index material.
In U.S. Pat. No. 4,457,590, U.S. Pat. No. 5,359,456, and U.S. Patent Application Publication No. 2008/0239510, although it is discussed that minute particles of metal or the like are mixed with the base material for attaining a distribution of refractive index or improving the refractive index, it is not discussed that the minute particles are mixed for the purpose of attaining a distribution of Abbe number.
U.S. Pat. No. 5,366,939 discusses an example of forming a distribution in which Abbe number is increased along with an increase in refractive index by distributing a first metal species with a gradient and distributing a second metal species substantially flatly in a medium.
However, since Abbe number variation Dvd attained with the configuration discussed in U.S. Pat. No. 5,366,939, which is 3.1, is small, a chromatic aberration correction effect thereof is not sufficient.